Cyclins are components of the core cell cycle machinery. They serve to activate the associated cyclin-dependent kinases (CDKs). This proposal focuses on cyclin A and its role in normal development and in neoplasia. Mammalian cells express two A-type cyclins: the ubiquitous cyclin A2 and testis-specific cyclin A1. According to our current understanding of the cell cycle machinery, cyclin A represents an essential S-phase cyclin which is required for all embryonic and somatic cell cycles in mammals. This notion is supported by the observation that cyclin A2-null mice die at embryonal day 5.5. Overexpression of cyclin A2 has been shown to play an important role in pathogenesis of many human cancers. Hence, cyclin A might represent a potential therapeutic target in human neoplasia. However, the current notion that cyclin A represents an essential component of the core cell cycle machinery has precluded exploring this possibility. We experimentally tested this notion by generating conditional cyclin A knockout mice (lacking both A-cyclins). This approach allowed us to bypass the early embryonic lethality and to derive cyclin A-null mouse embryonal fibroblasts. Surprisingly, we found that an acute shutdown of A-type cyclins in in vitro cultured fibroblasts had no impact on cell proliferation. Hence, contrary to the current notion, cyclin A is dispensable for normal, non-oncogenic proliferation at least in in vitro cultured fibroblasts. These unexpected findings raise several important issues: (1) What is the molecular basis of the S-progression in the absence of cyclin A? (2) Are A-type cyclins also dispensable for cell cycle progression in vivo, during normal mouse development? (3) Is cyclin A required for the oncogenic proliferation of cancer cells? In the proposed work we will address these issues. The overall hypothesis to be tested is that cyclin A is dispensable for normal, non-oncogenic proliferation of the majority of cell types, but it is critically required for oncogenic proliferation of at least a subset of cancers. If our hypothesis is confirmed, it will change our understanding of the cell cycle machinery, and it may lead to novel therapeutic approaches for cancer patients. In Aim 1 we will test our hypothesis that in mammalian cells A- type and E-type cyclins perform overlapping, redundant roles in driving S phase progression. Consequently, we hypothesize that cyclin A-deficient cells proliferate normally due to the presence of cyclin E. We further hypothesize that ablation of the E-type cyclins in cyclin A-null background would completely halt cell cycle progression. In Aim 2, we will use tissue-specific and inducible Cre strains to bypass the peri-implantational lethality, and to study the requirement for cyclin A function at later stages of development. Lastly in Aim 3, we will study the requirement for cyclin A function in Myc-driven transformation, and in breast and lung cancers in vivo. The Specific Aims are: Aim 1. To study the molecular basis of cyclin A function in cell cycle progression. Aim 2. To determine the requirement for cyclin A function in various lineages during normal development. Aim 3. To determine the requirement for cyclin A function in oncogenic cell proliferation. PUBLIC HEALTH RELEVANCE: The well-documented overexpression of cyclin A in a many human cancers suggests that cyclin A might represent a potential therapeutic target in human neoplasia. However, it is currently a textbook knowledge that cyclin A is essential for normal proliferation of all cells, thereby precluding exploration of anti-cyclin A therapeutic strategies. The work proposed in this application will likely change this notion, and will lead to re-focusing our efforts on cyclin A inhibition in human cancers. Hence, the proposed work will likely lead to novel therapeutic approaches for human cancer patients.